Allow parsing for enum case element parameter on new line

[kimdv]

Resolves #1709

[ahoppen]

CC @bnbarham @rintaro @DougGregor Any concerns with this PR? See #1709 for an explanation of the motivation.

[rintaro]

Since func/subscript parameter clause allows newlines before (, it's good to be consistent. Call arguments enforce ( be on the same line, but that is mainly for disambiguation.
TBH I personally don't want to allow newlines before ( even for func, but I couldn't find a strong reason to disallow them.

[DougGregor]

Since func/subscript parameter clause allows newlines before (, it's good to be consistent.

This is the argument that wins me over. I agree with @rintaro that I'd prefer not to allow newlines before the ( for any of func/decl/subscript/etc. However, since we already allow them, I'd rather be consistent and also allow them for cases. There's no chance for ambiguity here because there are no expressions within an enum scope.

[DougGregor]

@swift-ci please test

[ahoppen]

Thanks for your feedback @rintaro and @DougGregor. I agree with you as well, bug just wanted to double-check